A gallium nitride based-field effect transistor is a device in which high voltage resistance is expected from a size of band gap of a material. Actually, however, the high voltage resistance as expected is not achieved. One of the reasons is a problem of hole-withdrawing. When high electric field is applied to a transistor to drive the transistor, running electrons collide with crystal lattices, and this collision causes the phenomenon of a so-called impact ionization in which electrons and holes are generated. Electrons generated by the phenomenon are rapidly absorbed in a drain electrode. However, holes merely move relatively slowly toward the lower part of a source electrode in the crystal layer, and as a result, the holes remain and accumulate in crystals. When retention and accumulation of holes are caused, electrochemical potential of an element active layer is decreased, and more electrons run in the element active layer. This tends to induce further impact ionization. As a result, positive feedback phenomenon occurs, and then, a device leads to breakdown later. For this reason, when high voltage is applied to a transistor, the voltage should be applied to an extent such that positive feedback phenomenon of current increase does not occur. Even though band gap is large, it is not easy to achieve high voltage resistance commensurate with this.
The technology that holes generated by impact ionization are withdrawn outside an element active layer to overcome the problem is disclosed in, for example, JP 2001-168111A and JP 2004-342810A. This conventional technology is that in a GaN-based field effect transistor, a p-conductive GaN type epitaxial crystal layer electrically grounded is arranged over the entire lower part of the element active layer. It is considered that this constitution enable to withdraw holes generated by the phenomenon such as impact ionization outside from an element active layer by arranging the p-conductive type semiconductor crystal layer electrically grounded over the entire lower part of the element active layer.
According to the above conventional technology, a certain hole-withdrawing effect can be expected, but the technology has the following problems. A p-conductive GaN type crystal layer is generally formed doping magnesium or the like in high concentration, and therefore it has poor crystalline quality. As a result, an element active layer epitaxially grown on the crystal takes over poor crystalline quality of the p-conductive type gallium nitride layer. Due to this, the element active layer has problems that mobility of the running electrons is low, the problem that leakage current from a gate is large, and the like. Thus, it is difficult to prepare a transistor having practically usable level.
Furthermore, generally when a p-conductive type gallium nitride layer is formed, it is difficult to correctly and reproducibly control carrier concentration and distribution in the crystal, due to instability of activation rate and diffusion of a p-conductive type dopant. For this reason, the transistor having the structure that a p-conductive type layer is arranged at the lower part of a gate has another problem that fluctuation of threshold voltage and defective pinch-off often occur.